This invention relates to electrode leads for use in the detection and control of cardiac bradyarrhythmias and tachyarrhythmias, and particularly to a lead including cardioversion/defibrillation electrodes and pacing/sensing electrodes on a common lead body.
Research to provide an automatic implantable pacemaker/cardioverter/defibrillator has been in progress for over twenty years and has led to the implantation in recent years of several versions of such systems. Over the same period of time, considerable research and development effort has been expended in the development of cardioversion and defibrillation leads. In this context, both unipolar and bipolar pacing and sensing have been employed. Recently, it has also been disclosed that biphasic cardioversion and defibrillation pulses provide substantial benefits.
The traditional approaches to adding pacing and sensing electrodes to cardioversion/defibrillation leads require a separate conductor for each pacing and sensing electrode and for each independently usable defibrillation electrode. Thus, bipolar sensing typically requires at least three conductors, and in the case of multiple cardioversion/defibrillation electrodes, four or more conductors. In embodiments employing multiple cardioversion/defibrillation electrodes, even unipolar sensing correspondingly typically requires at least three conductors. As the size and complexity of these leads increases with each additional conductor, a reduction of the number of conductors per lead is desirable.
In endocardial leads, a further complication arises. It is believed desirable in many cases to locate a right ventricular cardioversion/defibrillation electrode as close to the apex of the heart as possible. However, placement of one or two sense/pace electrodes at the distal end of the lead (a preferred location) typically results in location of the defibrillation electrode in a less apical location, in order to provide space for the pace/sense electrode or electrodes.
Bipolar sensing has been disclosed to be particularly effective in the detection of "near field" ECGs which are used for arrhythmia detection and or synchronization of delivered cardioversion and defibrillation shocks in some current devices. While workable, it is believed less than optimal to employ a small surface area pace/sense electrode in conjunction with a large surface area cardioversion/defibrillation electrode for sensing the near field ECG. The large surface area cardioversion electrode may extend over or near enough to conduction pathways which reflect both atrial and ventricular originated ECG components. If large enough, the "far field" components can in some cases be detected and confuse the tachyarrhythmia detection circuitry and algorithm of the pulse generator. Consequently, a number of references recommend the use of a separate pair of closely spaced, small surface area pace/sense electrodes for sensing. For example, see U.S. Pat. No. 4,614.192, issued to Imran et al., and U.S. Pat. No. 5,044,375, issued to Bach, et al. It should also be Noted that Implantable pacemaker/cardioverter/defibrillators currently in clinical testing, manufactured by Medtronic, Inc., employ ventricular endocardial leads as generally illustrated in U.S. Pat. No. 5,014,696, issued to Mehra, which include a bipolar electrode pair for sensing, located adjacent the distal end of the lead, and a large surface area coil electrode located proximal to the bipolar pair.
U.S. Pat. No. 4,440,172 describes a number of embodiments of pacing and cardioversion electrodes and leads which provide for unipolar pacing and sensing through the use of endocardial ventricular tip electrodes or an epicardial button electrodes paired with epicardial defibrillation electrodes. In one embodiment, a combined defibrillator-pacer system is depicted where the monophasic defibrillation shocks and pacing pulses are transmitted over the same conductor pair through the lead body but are routed to different electrodes on the basis of their polarity using steering diodes located within the lead. However, this approach would appear to be problematic in the event that biphasic cardioversion/defibrillation pulses were to be employed.
In U.S. Pat. No. 4,999,907, a transvenous, endocardial version lead is described which employs a pair of distal electrodes and a pair of proximal electrodes, which are respectively intended to be lodged in the ventricular apex of the right ventricular chamber of the heart and the superior vena cava. Each pair of electrodes is intended to be electrically connected in common by circuitry within the implantable cardioverter during cardioversion. However during sensing and pacing, the closely spaced distal ventricular electrodes are connected to an ECG sense amplifier and a pacing energy pulse generator. The lead also includes circuitry for limiting the voltage of shocks delivered by the lead. U.S. Pat. No. 4,595,009 discloses switching circuitry which controls the interconnection of the pulse generators, sense amp and electrodes during pacing, sensing and cardioversion in conjunction with a lead generally as illustrated in the '907 patent, but lacking the voltage limiting circuitry.
One additional proposed approach to reducing the numbers of lead conductors in cardioversion and defibrillation leads has been to allow a sensing or pacing electrode to share a conductor with a cardioversion or defibrillation electrode by inserting back-to-back zener diodes between the sensing or pacing electrode and the cardioversion or defibrillation electrode. Examples of such leads may be found in U.S. Pat. No. 5,336,253, issued to Mehra et al. and incorporated herein by reference in its entirety.